The groundwork for mitochondrial medicine was laid 30 years ago with identification of the first disease-causing mitochondrial DNA (mtDNA) mutations in 1988. Three decades later, mutations in nearly 300 genes involving every possible mode of inheritance within both nuclear and mitochondrial genomes are now recognized to collectively comprise the largest class of inherited metabolic disease, affecting at least 1 in 4300 individuals across all ages. Significant progress has been made in recent years to improve understanding of mitochondrial biology and disease pathophysiology.